Electromechanical transducer and method for manufacturing the same

ABSTRACT

The present invention provides a technology for decreasing a dispersion of the performance among electromechanical transducers each having through wiring. A method for manufacturing an electromechanical transducer includes: obtaining a structure in which an insulative portion having a through hole therein is bonded onto an electroconductive substrate; filling the through hole with an electroconductive material to form a through wiring which is electrically connected with the electroconductive substrate; and using the electroconductive substrate as a first electrode, forming a plurality of vibrating membrane portions including a second electrode, which opposes to the first electrode through a plurality of gaps, on an opposite side of the first electrode to the side having the insulative portion, to thereby forming a plurality of cells.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electromechanical transducer to beused for an ultrasound probe of an ultrasound diagnosis apparatus andthe like, and to a method for manufacturing the same.

2. Description of the Related Art

An electromechanical transducer such as an ultrasound transducerconducts at least one of conversion from an electric signal to anultrasound wave and conversion from the ultrasound to the electricsignal, and it is used as a probe for ultrasound diagnosis for medicalapplication, a probe for a non-destructive test and the like. In recentyears, along with the development of a fine processing technology, acapacitance type electromechanical transducer (CMUT: CapasitiveMicromachined Ultrasonic Transducer) produced by using the technologyhas been actively developed. An exemplary CMUT has a structure that anelement substrate having a cell includes a substrate having a lowerelectrode (where the substrate occasionally serves as lower electrode aswell), a vibrating membrane formed on the substrate so as to keep afixed space between itself and the lower electrode, and an upperelectrode, which element substrate being electrically connected with adriving circuit substrate. Furthermore, the CMUT has a structure inwhich the element substrate has a plurality of elements to which two ormore cells are electrically connected, which element substrate beingelectrically connected to the driving circuit substrate (see JapanesePatent Application Laid-Open No. 2006-319712). A CMUT with high abilityin transmitting or receiving the ultrasound by using a lightweightvibrating membrane, and having excellent broadband characteristics inany of a liquid and an air, can be easily obtained. Consequently, sincethe use of CMUT enables diagnosis with higher accuracy than that of aconventional medical diagnosis, and CMUT has received attention as apromising technology.

In operating CMUT, when transmitting the ultrasound, the CMUT applies aDC voltage and an AC voltage between the lower electrode and the upperelectrode so that the voltages are superimposed. Thereby, the vibratingmembrane vibrates and transmits the ultrasound. When receiving theultrasound, the CMUT detects a signal from a change in the capacitancebetween the lower electrode and the upper electrode due to the change ofa distance between both of the electrodes along with the deformation ofthe vibrating membrane, which occurs when the vibrating membrane hasreceived the ultrasound. The methods for applying voltage to electrodesfor driving the CMUT include: providing an electrode on the surface ofthe substrate of the CMUT and routing wires which are connected to theupper and lower electrodes; and guiding wires from the upper and lowerelectrodes to the rear surface of the substrate by using through wiringprovided on the substrate, and thereby electrically connecting theelectrodes to each other. In the former method, since it is necessary toroute the wires on the surface of the substrate, an element can hardlybe arranged in a portion occupied by the wire. Therefore, a fill factor,which is expressed by a ratio of the cells occupying in the elementshaving the same area, decreases. In addition, as for the space betweenthe elements, since it is necessary to arrange the elements so as to beseparated from each other by the area occupied by the wire, the elementscan hardly be arranged at a high density. As a result, the performanceof the CMUT is lowered. On the other hand, in the method of using thethrough wiring, normally, every element is insulated from each other inthe substrate, and the through wiring is formed for the respectiveelements to electrically connect the electrodes. Thus manufactured CMUTis described in Japanese Patent Application Laid-Open No. 2007-215177and Japanese Patent Application Laid-Open No. 2010-35134. Since there isneed to route wires on the surface of the substrate in the CMUT formedby using the through wiring, the cell can be arranged in the portionsoccupied by the wires and the element can be arranged at a high density.Therefore, the CMUT having a high fill factor and a high arrangementdensity of the element can be produced, which leads to the enhancementof the performance.

SUMMARY OF THE INVENTION

In the CMUT of Japanese Patent Application Laid-Open No. 2007-215177,through wiring is formed in the substrate of the CMUT. As a techniquefor forming the through wiring in the substrate, there are a method offorming the through wiring in the substrate before the element of theCMUT is prepared, and a method of forming the through wiring in thesubstrate after the element of the CMUT has been formed. However, in theformer method, generally, the formation of the through wiring in thesubstrate leads to, on the surface of the substrate, formation of stepsbetween the substrate and the through wiring. Even if it is tried toflatten the surface with a Chemical Mechanical Polishing (CMP) processor the like, there is a limitation in flatness of the surface of thesubstrate due to a difference between the materials. When the cells arearranged on the substrate having the steps, the lower electrode, thegap, the vibrating membrane and the upper electrode occasionally resultin being unevenly formed as a result of receiving the influence of thesteps. Consequently, characteristics of the cell become differentbetween a portion having the through wiring therein and a portion havingno through wiring therein, which leads to the lowering of theperformance of the CMUT. On the other hand, in the latter method, sincethe through wiring is prepared after the element substrate has beenprepared, it is difficult to arrange the cell right on the throughwiring because of limitation by the manufacturing process. Furthermore,since the cell is formed of a thin film which is structurally weak,there is a possibility that a yield decreases in an operation of formingthe through wiring.

In the method of Japanese Patent Application Laid-Open No. 2010-35134,the CMUT having the through wiring is formed by forming the elements inthe CMUT and then bonding a substrate having the through wiring formedtherein to the element substrate. In this technique as well, thesubstrate of the through wiring is bonded to the element substrate aftera structure having fine gaps therein has been formed, and accordinglythere is a need that a yield of the element be increased in an operationof bonding the through wiring substrate and an operation after thebonding operation.

With respect to the above described problems, a method for manufacturingan electromechanical transducer includes: obtaining a structure in whichan insulative portion having a through hole therein is bonded onto anelectroconductive substrate; filling the through hole with anelectroconductive material to form a through wiring which iselectrically connected with the electroconductive substrate; and usingthe electroconductive substrate as a first electrode, forming aplurality of vibrating membrane portions including a second electrode,which opposes to the first electrode through a plurality of gaps, on anopposite side of the first electrode to the side having the insulativeportion, to thereby forming a plurality of cells.

With respect to the above described problems, the electromechanicaltransducer of the present invention has a configuration including: aplurality of cells which are formed by mounting a plurality of vibratingmembrane portions including a second electrode, which opposes to a firstelectrode through a plurality of gaps, on a side of the first electrodeof an electroconductive substrate, in which the first electrode has aninsulative portion bonded onto an opposite side of the side having thegaps, and the insulative portion has a through wiring formed thereinwhich is electrically connected to the first electrode.

According to the present invention, an electromechanical transducer isformed while using an electroconductive substrate as an electrode, afterhaving ensured an electric connection between the electroconductivesubstrate and a through wiring, by obtaining a structure in which aninsulative portion having a through hole is bonded onto theelectroconductive substrate, and then filling the through hole with anelectroconductive material to form the through wiring. Consequently, theelectroconductive substrate can be used as the electrode in such a statethat the flatness of the electroconductive substrate is not impaired,and consequently by using the electroconductive substrate of which theprecision of the flatness has been guaranteed, the cell can be arrangedeven right on the through wiring, without being affected by a stepbetween the through wiring and the insulative portion. Consequently,since the cell can be arranged even right on the through wiring, thenumber of cells arranged in the element of the electromechanicaltransducer can be increased, thereby the fill factor is enhanced, whichleads to the enhancement of the performance. In addition, the dispersionamong the cells can be decreased. Furthermore, since the cell which isstructurally weak due to fine gaps therein is formed after the throughwiring has been formed, the lowering of the yield can be reduced.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views illustrating a cross section and an uppersurface of a CMUT which is an electromechanical transducer according tothe present invention.

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H, 2I, 2J, 2K, 2L and 2M arecross-sectional views for describing a process flow of ExemplaryEmbodiment 1 of a method for manufacturing the electromechanicaltransducer.

FIGS. 3A, 3B, 3C, 3D and 3E are cross-sectional views for describing aprocess flow of Exemplary Embodiment 2 of the method for manufacturingthe electromechanical transducer.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

In the present invention, the electromechanical transducer is configuredto have a through wiring that is formed by producing a structure inwhich an insulative portion having a through hole therein is bonded ontoan electroconductive substrate, and then filling the through hole withan electroconductive material, the through wiring being electricallyconnected to a electroconductive substrate, and have a plurality ofcells formed on the electroconductive substrate. Based on suchconception, the electromechanical transducer of the present inventionand the method for manufacturing the same have a basic constitution aswas described in the section of the summary of the invention. Thestructure can be obtained by a method which includes bonding aninsulative substrate that is the insulative portion having the throughhole formed therein onto the electroconductive substrate, as will bedescribed in Exemplary Embodiment 1 which will be described later.Another method can also be adopted which includes forming aphotosensitive insulative portion on the electroconductive substrate tobond the insulative portion with the electroconductive substrate, andforming a through hole which reaches the electroconductive substrate inthe photosensitive insulative portion, as will be described in ExemplaryEmbodiment 2 which will be described later.

Furthermore, when the second electrode is used in common for a pluralityof elements each containing at least one cell, the method may alsoinclude electrically separating the first electrode for every element.In this case, the method may also include forming a portion that is aportion of the electroconductive substrate, which has been electricallyseparated from the first electrode, and which is electrically connectedto any one of the through wirings, and electrically connecting thecommon second electrode to the portion. On the other hand, the firstelectrode can also be made common for the plurality of the elements. Inthis case, a plurality of portions that are a plurality of portions ofthe electroconductive substrate can be formed, which are electricallyseparated from the first electrode, and that are electrically connectedto each corresponding through wiring, electrically separate the secondelectrodes for every element, and electrically connect each of thesecond electrodes to each of the portions. In addition, the vibratingmembrane portion may be formed so as to have the vibrating membraneprovided through a gap and the second electrode formed on the vibratingmembrane, or it may be formed of an electroconductive vibrating membranewhich serves also as the second electrode.

The electromechanical transducer and the method for manufacturing thesame to which the present invention can be applied will be describedbelow with reference to the drawings, while taking CMUT as an example.Incidentally, the same portions in between the following exemplaryembodiments will be designated by the same reference numerals, so thatthe description will be simplified.

FIGS. 1A and 1B are schematic views illustrating a structure of CMUT ofan exemplary embodiment which is manufactured with a manufacturingmethod of the present invention. FIG. 1A is a cross-sectional view takenalong the line 1A-1A of FIG. 1B; and FIG. 1B is a top plan view of CMUT.The CMUT which has been manufactured with the manufacturing method ofthe present invention includes a substrate 1, an insulative substrate ora member 3 having a through wiring 4 therein, and a CMUT device formedon the substrate 1. The CMUT device includes: a vibrating membrane 8which is formed on the substrate 1 that serves also as a firstelectrode, through a gap 7 (approximately vacuum void, void filled withgas, or the like); a second electrode 9; and a cell 11 including asealing film 10. Here, the vibrating membrane 8, the second electrode 9and the sealing film 10 constitute a vibrating membrane portion. Inaddition, the CMUT device has a large number of elements to which aplurality of cells 11 are electrically connected, and the elements aretwo-dimensionally arrayed. In each element, one or more (one inillustrated example) through wirings 4 are connected to the substrate 1which becomes a common first electrode. Here, the second electrode 9becomes a common electrode for all of the elements. In the presentexemplary embodiment, this common second electrode 9 is electricallyconnected to a portion of the substrate 1 of the first electrode, whichhas been electrically separated from the substrate, and to the throughwiring (a portion of substrate 1 in leftmost portion of FIG. 1A andthrough wiring 4), and the electric current is taken out to the outside.In the present exemplary embodiment, the substrate 1 which is the firstelectrode is individually electrically separated so as to correspond toeach element, and the second electrode 9 is used as the common electrodefor all of the elements. However, as described above, the substrate 1 ofthe first electrode may be used as a common electrode, and the secondelectrode 9 may be individually separated so as to correspond to eachelement.

The method for manufacturing the CMUT of the present exemplaryembodiment includes bonding an insulative substrate or member onto anelectroconductive substrate 1 which serves also as a first electrode,forming a through wiring in the insulative substrate to ensure anelectric connection with the substrate 1, and forming anelectromechanical transducer by using the substrate 1 as the firstelectrode. In other words, by connecting the insulative substrate havingthe through wiring therein, to the rear surface of the substrate 1beforehand, the CMUT can be formed on the substrate 1 in such a statethat the flatness of the surface of the substrate 1 is kept. Therefore,if an Si substrate which has the flatness guaranteed and has a lowresistivity is used as the substrate 1, it is suppressed that a stepwhich tends to be easily formed when the through wiring is formed occurson the surface of the substrate, and the CMUT can be formed withoutbeing affected by the step, which can enhance the consistency of theperformance of the CMUT. In addition, since the cell 11 of the CMUT canbe formed even right on the through wiring, a fill factor can beenhanced, and the performance of the CMUT can be enhanced. Furthermore,the through wiring is not formed after the CMUT that has a fine gaptherein and thus that is structurally weak has been formed, which canconsequently decrease the lowering of the yield occurring when thethrough wiring is formed.

When the CMUT is formed on a structure having the through wiring therein(which means an object formed by combining the substrate 1 with aninsulative substrate or member), a sacrificial layer 13 for forming agap 7 is film-formed and patterned on the substrate 1 which serves alsoas the first electrode, and a vibrating membrane 8, for instance, isformed on the substrate 1. Furthermore, a second electrode 9 isfilm-formed and patterned. Next, an etching hole for removing thesacrificial layer 13 is formed through the vibrating membrane 8, and thegap 7 is formed by selectively removing only the sacrificial layer 13.Then, a sealing film 10 is formed to seal the above described etchinghole for removing the sacrificial layer, and then an electromechanicaltransducer such as the CMUT is produced.

As the substrate 1 to be used in the present exemplary embodiment, an Sisubstrate is desirable of which the flatness of the surface can beguaranteed (in other words, surface roughness is small value) and whichis easy to be subjected to fine processing. In addition, since thesubstrate 1 serves also as the first electrode, the resistivity of thesubstrate 1 is desirably 0.02 Ωcm or less. This is because the firstelectrode having a smaller wiring resistance can more decrease a loss ofa signal. Furthermore, the substrate 1 desirably has a surface roughnessRms of 0.5 nm or less. This is because since the CMUT is formed bystacking thin films on the substrate 1, as the surface roughness of thesubstrate 1 is smaller, the CMUT having smaller dispersion can beformed.

The substrate 1 can employ an active layer of an Silicon on Insulator(SOI) wafer which is produced so as to contain an SiO₂ film between Siand Si. Normally, after an element separating groove 6 has been formed,it is necessary to film-form the second electrode 9 which is the upperelectrode while showing adequate coverage so that the second electrode 9surmounts a stage originating in a step formed between the substrate 1and the insulative portion 3 by the groove 6. However, when thesubstrate 1 is formed by using an SOI active layer, the substrate 1 canbe thinly formed, and accordingly the step to be formed by the elementseparating groove 6 becomes small, which enhances the stability of theprocess.

The material for the sacrificial layer 13 to be used in themanufacturing method in the present exemplary embodiment can be selectedfrom materials which have etching selectivity with respect to thevibrating membrane 8 and which have sufficient heat resistance such thatthe surface roughness is not largely changed by heat when the vibratingmembrane 8 is formed. The material is desirably, for instance, Cr or Mo.The material for the second electrode (upper electrode) 9 to be used inthe manufacturing method in the present exemplary embodiment may beselected from materials which have sufficient heat resistance such thatthe surface roughness is not largely changed by heat in heat treatmentto be conducted when the sealing film 10 is formed, and which haveetching selectivity with respect to the sacrificial layer 13 when thesacrificial layer 13 is etched. For instance, materials such as Ti, W,TiW and Mo can be selected. In addition, the material for the vibratingmembrane 8 to be used in the manufacturing method in the presentexemplary embodiment is desirably an SiN film of which the stress can becontrolled and which is excellent in mechanical characteristics andinsulation performance and which is film-formed with PECVD(plasma-enhanced chemical vapor deposition).

Since the sealing film 10 is used also as a vibrating membrane portion,such a material can be selected that not only the film is formed on asealing portion while showing adequate coverage, but also the stress canbe controlled and the film has excellent mechanical characteristics andinsulation performance. For instance, an SiN film which can befilm-formed with the PECVD can be selected. The material for theinsulative substrate or member which forms the through wiring therein inthe present exemplary embodiment needs to be a material which is easy toform a through hole that becomes the through wiring and which can bebonded to the substrate 1. For instance, Pyrex (registered trademark)glass or quartz glass can be used. The quartz glass or the Pyrex(registered trademark) glass can have a fine through hole formed thereinwith a sand blast process or the like, and they can be easily bonded,for instance, to an Si substrate which becomes the substrate 1.Particularly, the Pyrex (registered trademark) glass has an advantage ofshowing high compatibility with the Si substrate in the heat treatment,because of having a close coefficient of thermal expansion extremely tothat of Si which becomes the substrate 1.

Furthermore, a photosensitive resin and a glass material can also beused as an insulative member which forms the through wiring therein. Thephotosensitive resin material which can be applied with a spin coatingmethod can easily form an applied film, for instance, on the Sisubstrate, and can also form the through hole therein with aphotolithographic technology. In addition, there exist also a materialwhich is excellent in resistance to heat generated when theelectromechanical transducer such as the CMUT is formed, andphotosensitive polyimide (manufactured by Toray Industries, Inc.), forinstance, can be used. Furthermore, a photosensitive dry film(commercial product manufactured by Hitachi, Ltd., Asahi KaseiCorporation, TOKYO OHKA KOGYO CO., LTD. or the like) can also be used asthe insulative substrate or member.

A further specific example of a method for manufacturing anelectromechanical transducer, to which the present invention can beapplied, will be described in detail below with reference to thedrawings.

Exemplary Embodiment 1

The exemplary embodiment 1 is concerned to a method for manufacturingthe CMUT that is produced on a substrate which is formed of alow-resistance Si substrate and an insulative glass substrate and towhich a through wiring is connected. Although a Pyrex (registeredtrademark) glass is used as an insulative substrate 3 provided with athrough hole in the present exemplary embodiment, a basic manufacturingmethod is the same even when another material such as quartz glass isused.

FIGS. 2A to 2M are views for describing a process flow of the presentexemplary embodiment. Although a cross section of a device having twoelements is illustrated in these views for simplification of thedrawings, other elements can also be produced in a similar way. Firstly,an Si substrate which becomes a substrate 1 is prepared (FIG. 2A). Sincethe substrate 1 serves also as a first electrode which becomes a lowerelectrode, it is desirably a substrate having a low resistivity. In thepresent exemplary embodiment, an Si substrate having a resistivity of0.02 Ωcm or less is used. In addition, since the CMUT is formed on thesurface of the substrate 1, the substrate having a small surfaceroughness Rms as 0.5 nm or less is used. Next, a thermally-oxidized filmlayer of approximately 1 μm is formed on both surfaces of the substrate1 as an insulating film 15. Since the thermally-oxidized film to beformed on the Si substrate is excellent in flatness, the insulating film15 can be formed almost without impairing the flatness of the Sisubstrate. Because the vibrating membrane 8 as described later hasinsulating properties, if an electrical isolation between upper andlower electrodes can be sufficiently ensured, the insulating film 15 canbe omitted.

Next, in order to bring the through wiring portion into ohmic contactwith the substrate 1, an oxide film on the rear surface of the Sisubstrate is stripped, and an ohmic metal 2 is film-formed andpatterned. Furthermore, a layer for having the ohmic contact with the Sisubstrate 1 is formed by being subjected to an annealing treatment. Ametal such as Al and Ti which is easy to form an alloy layer with Si isused as a metal for having the ohmic contact (FIG. 2B).

Next, the Pyrex (registered trademark) glass as the insulative substrate3 provided with the through hole, in which the through hole has beenformed with a sandblast method in a portion at which the through wiringis to be formed, is bonded to the rear surface of the substrate 1 sothat an ohmic metal 2 overlaps with the through hole (FIG. 2C). ThePyrex (registered trademark) glass and the Si substrate can be bonded toeach other by anode bonding or direct bonding. Although the through holewas formed with the sandblast method in the present exemplaryembodiment, the method of forming the through hole is not limited to thesandblast method. The through hole can be formed also with a drillingmethod, a laser machining method, an ultrasonic machining method, andfurther a method of combining a photolithographic technology with a dryetching technique. In addition, the bonding method between the Pyrex(registered trademark) glass and the Si substrate were, where anodicoxidation is used herein, is not limited to the thereto, and otherbonding methods such as direct bonding can also be employed.

The through hole is filled with an electroconductive material, whichbecomes the through wiring. Cu by plating is used for theelectroconductive material to fill the through hole (FIG. 2D). After thethrough wiring has been formed, the surface heights of the throughwiring 4 and the Pyrex (registered trademark) glass 3 are equalized bypolishing the rear surface with CMP. Next, a protective material 12 forprotecting the metal which forms the through wiring, in various etchingtreatments in the processes, is formed on the rear surface of the bondedsubstrate. In the present exemplary embodiment, Ti having highselectivity in etching and high resistance to heat is film-formed so asto have a thickness of 300 nm with an EB vapor deposition method. Bythis step, a structure of the substrate having the through wiring 4formed therein is completed (FIG. 2E).

Next, a process of forming CMUT on the structure of the producedsubstrate will be described below (FIG. 2F to FIG. 2M). In the methodfor manufacturing the CMUT of the present exemplary embodiment, a gap isformed with by: patterning beforehand the material which is referred toas a sacrificial layer; then forming a vibrating membrane thereon; andselectively removing the sacrificial layer. Thereby, theelectroconductive substrate is used as a first electrode, a plurality ofvibrating membrane portions including a second electrode opposing to thefirst electrode through a plurality of gaps, respectively, are formed,on the first electrode on an opposite side of the side having theinsulative portion, and thus a plurality of cells can be formed. Sincethe CMUT to be produced with this process can be produced at acomparatively low temperature of 350° C. or lower, it can be formed evenwith a method for forming the CMUT after the through wiring 4 has beenformed with decreased affection by the heat.

Firstly, a resist is applied onto the surface of the prepared substratehaving the through wiring 4 and it is patterned with a photolithographictechnology. The resist pattern is used as a mask to subject thesubstrate 1 as a lower electrode to dry etching by using SF₆ as anetching gas, until the substrate 1 is cut down to the face at which aninsulative substrate 3 and the substrate 1 are bonded. Thereby, anelement separating groove 6 is formed. By this operation, every portionof the substrate 1, which corresponds to each element, is electricallyseparated from each other (FIG. 2F).

Subsequently, the sacrificial layer 13 is formed and patterned on thesubstrate 1 (FIG. 2G). The sacrificial layer 13 film-formed of Cr isproduced with the EB vapor deposition method. The pattern of thesacrificial layer 13 can be formed by lowering the vacuum degree to2.0×10⁻⁴ Pa, film-forming Cr of approximately 200 nm with the EB vapordeposition method, then forming a resist mask with the photolithographictechnology, and etching Cr with an etchant of a chromic acid mixture(chromium etchant, made by KANTO CHEMICAL CO., INC.).

Next, the vibrating membrane 8 is formed on the sacrificial layer 13(FIG. 2H). In this step, the vibrating membrane 8 is formed with aplasma CVD method to form an SiN film which becomes the vibratingmembrane 8. The SiN film with a film thickness of approximately 440 nmcan be formed in a mixed gas of SiH₄, N₂ and NH₃ at a substrate heatingtemperature of 350° C. and a chamber pressure of 1.6 Torr forapproximately 200 seconds, and subsequently the vibrating membrane andan insulating film 15 corresponding to an extraction portion of theupper electrode are etched with a dry etching technique using CF₄ as anetching gas, in order to form the portion at which the second electrode9 that becomes the upper electrode is connected with the through wiring4 (FIG. 2I).

Next, the second electrode 9 which becomes the upper electrode is formed(FIG. 2J). A Ti film which becomes the upper electrode can be formedwith an EB vapor deposition method. The Ti film of 100 nm is formed witha vacuum degree of 2.5×10⁻⁴ Pa. Furthermore, a resist mask is formedwith a photolithographic technology. In addition, a resist forprotecting Ti of the rear surface is formed. Furthermore, a pattern ofthe upper electrode 9 is formed by etching Ti with a Ti etchant (WLC-T,made by MITSUBISHI GAS CHEMICAL COMPANY, INC.).

Subsequently, the sacrificial layer 13 is removed, which becomes the gap7 (FIG. 2K). Firstly, a resist mask which has been formed with thephotolithographic technology is provided, and an etching hole foretching the sacrificial layer 13 is formed in the vibrating membrane 8with dry etching in which CF₄ is used as an etching gas. In FIG. 2K, theetching hole is omitted for the simplification of the drawing. Then, thesacrificial layer 13 is selectively removed through the etching hole, byimmersing the substrate into the etchant of the chromic acid mixture,which is the etchant for the sacrificial layer 13, thus the gap 7 isformed. After the sacrificial layer 13 has been completely removed, thegap is sufficiently washed with water, the water is replaced byisopropyl alcohol, the gap is finally dried with a fluorine-based lowsurface tension solvent (HFE7100, made by Sumitomo 3M Limited), andthereby the gap 7 is formed.

Next, the etching hole which has been used for forming the gap 7 issealed. The sealing film 10 is formed on the vibrating membrane 8 inwhich the etching hole has been formed, with a plasma CVD method (FIG.2L). An SiN film 10 with a film thickness of approximately 700 nm can beformed by film formation in a mixed gas of SiH₄, N₂ and NH₃ at asubstrate heating temperature of 350° C. and a chamber pressure of 1.6Torr for approximately 320 seconds. The gap 7 which has been sealed inthis operation becomes a void having an approximately equal pressure tothe pressure of the above described chamber.

Finally, a Ti film for protection the rear surface is removed, and anunder bump metal is formed under the through wiring 4 (FIG. 2M). In theoperation of removing the Ti film, Ti is etched by a Ti etchant (WLC-T,made by MITSUBISHI GAS CHEMICAL COMPANY, INC.) which has etchingselectivity for the Ti film but not having etching selectivity for theCu that forms the through wiring. Thereby, only the Ti film (protectivematerial 12) on the rear surface can be removed while a selection ratioof the Ti film to Cu is kept. The CMUT is completed by further formingan Au/Ni/Ti film which becomes the under bump metal 5 on a portion ofcoming in contact with the through wiring 4, by using a stencil mask.

By being produced in the above described process, an electromechanicaltransducer can be produced/developed on a structure of a substratehaving a flat surface, without being affected by a step between thethrough wiring and the substrate, which occasionally occurs when thethrough wiring is formed, and consequently the electromechanicaltransducers of which the dispersion of performance has been decreasedcan be produced.

Exemplary Embodiment 2

In a manufacturing method of Exemplary Embodiment 2, an insulativemember which is an insulating layer is formed by applying a resin or aglass material having photosensitive characteristics onto alow-resistance Si substrate. Then, by patterning a through hole in theinsulative member, a structure of a substrate to which a through wiringis connected is prepared, and CMUT is produced on the structure.

In the present exemplary embodiment, polyimide (commercially availableproduct made by Toray Industries, Inc., Asahi Kasei Corporation, HitachiChemical Company, Ltd. or the like) is used as a photosensitive resinmaterial. Although the polyimide is used here as the photosensitiveresin material, KI-1000 series (made by Hitachi Chemical Company, Ltd.),TMMR (made by TOKYO OHKA KOGYO CO., LTD.), SU-8 (made by KayakuMicroChem Corporation) and the like can also be used. Furthermore, aphotosensitive dry film (commercial product made by Hitachi, Ltd., AsahiKasei Corporation, TOKYO OHKA KOGYO CO., LTD. or the like) can also beused. However, in the operation of film-forming an SiN film of avibrating membrane 8 to be film-formed with a plasma CVD method, it isnecessary to adjust film formation conditions according to theheat-resistant temperature of each resin material. Although a filmformation temperature of the vibrating membrane 8 is set at 350° C. inthe present exemplary embodiment, if the film formation temperature is300° C. or higher, the vibrating membrane having adequate mechanicalperformance and insulation performance can be formed.

FIGS. 3A to 3E illustrate a process flow of the present exemplaryembodiment. Firstly, in a process of connecting the through wiring tothe substrate, a substrate 1 illustrated in FIG. 3A is used in thepresent exemplary embodiment, which is a Si substrate having lowresistivity. Since the substrate 1 serves also as a first electrode, itis desirable that the substrate 1 has small surface roughness and lowresistivity, similarly to that in Exemplary Embodiment 1. Specifically,it is desirable that the substrate has an Rms of 0.5 nm or less and aresistivity of 0.02 Ωcm or less.

Subsequently, an insulating layer 15 which is a thermal oxide film isformed on the substrate 1 so as to be 1 μm, and an ohmic metal 2 forhaving an ohmic contact is further formed on the rear surface of thesubstrate by film formation and patterning. (FIG. 3B). These operationsare similar to those in FIG. 2A and FIG. 2B of Exemplary Embodiment 1.Next, a photosensitive polyimide solution which becomes an insulativemember 14 provided with the through wiring is uniformly applied onto therear surface of the substrate 1 so as to have a thickness of 50 μm witha spin coating method, and is heated and dried on a hot plate (FIG. 3C).Furthermore, the solution is exposed to light through a photomask with aphotolithographic technology and it is developed with a developer of2.38% TMAH (tetramethyl ammonium hydroxide), and thereby the throughhole is formed so as to correspond to the ohmic metal 2. Subsequently,the through hole for the through wiring 4 is provided in the insulativemember by heating and curing the applied film in a nitrogen atmosphereof 300° C. so as to promote imidization (FIG. 3D).

When the through hole provided as described above is filled with a metalin a similar operation to that of FIG. 2D of Exemplary Embodiment 1(FIG. 3E), a structure of the substrate to which the through wiring 4 isconnected is formed by operations down to the present operation. As forthe following operations, similar operations to those of FIG. 2E to FIG.2M of Exemplary Embodiment 1 are conducted on the structure of thesubstrate, and then CMUT having the through wiring 4 is formed.

By the above described operations as well, an electromechanicaltransducer can be produced/developed on the structure of the substratehaving a flat surface without being affected by the step between thethrough wiring and the substrate, which occasionally occurs when thethrough wiring is formed, and accordingly the electromechanicaltransducers of which the dispersion of performance has been decreasedcan be produced.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2011-186731, filed Aug. 30, 2011, which is hereby incorporated byreference herein in its entirety.

1.-8. (canceled)
 9. An electromechanical transducer comprising: aplurality of cells which are formed by mounting a plurality of vibratingmembrane portions including a second electrode, which opposes to a firstelectrode through a plurality of gaps, on a side of the first electrodeof an electroconductive substrate, wherein the first electrode has aninsulative portion bonded onto an opposite side of the side having thegaps, and the insulative portion has a through wiring formed thereinwhich is electrically connected to the first electrode.
 10. Theelectromechanical transducer according to claim 9, wherein the firstelectrode is separated for every element containing at least one cell,and the insulative portion has a plurality of through wirings which areelectrically connected to the first electrodes which are separated forevery element.
 11. The electromechanical transducer according to claim9, wherein the electroconductive substrate is a silicon substrate. 12.The electromechanical transducer according to claim 9, wherein thevibrating membrane comprises an SiN film.
 13. The electromechanicaltransducer according to claim 9, wherein the insulative portion iscomposed of a photosensitive resin or a glass material.
 14. Theelectromechanical transducer according to claim 9, wherein an ohmicmetal is provided between the electroconductive substrate and thethrough wiring.
 15. The electromechanical transducer according to claim9, wherein the through wiring is composed of copper.